Umbrella base



FIG. 1 is a front elevation view of an umbrella base;

FIG. 2 is a rear elevation view thereof;

FIG. 3 is a left side elevation view thereof;

FIG. 4 is a right side elevation view thereof;

FIG. 5 is a top plan view thereof;

FIG. 6 is a bottom plan view thereof; and,

FIG. 7 is a perspective view thereof.

The broken lines depict portions of the article that form no part of the claimed design. 

CLAIM The ornamental design for an umbrella base, as shown and described. 